Chapter 10. Gamma Correction and Precision Color

Contents:

Anyone who has transferred images between a PC and a Macintosh--or even
simply viewed on one platform an image created on another--has probably
noticed one of the little gotchas of the computer world: images don't
look the same on all systems. Images created on Macs tend to look too
dark on PCs; images created on PCs tend to look too bright and washed out
on Macs. A pure yellow on one machine may have an orange or greenish tint
on another. Even on a single machine there are usually obvious changes in
brightness and color as the monitor (CRT) warms up, not to mention when
the user adjusts the screen controls. And in the absence of tedious
calibration procedures and high-end color-conversion software, what comes
out of the printer is, at best, only a vague approximation of what the
screen shows.

PNG certainly doesn't solve all of these problems, but it does provide
image authors with the means to minimize many of them, as long as the
editing and viewing software is written properly. As recently
proposed standards are approved and implemented in hardware, from
graphics cards, to monitors, to printers and scanners, there is reason
to expect that platform-independent color will become the norm, not
the exception, in the new millennium.

10.1. Transfer Functions and Gamma

To understand the solutions, one must first become acquainted with the
problems. I won't attempt to cover the subject in detail; an entire
book could be written on it--and, indeed, Charles Poynton has done
just that.
But I will give a brief overview of the main issues and explain how
some of the features of the Portable Network Graphics format fit into
the picture. I may even mention some physics and an equation or
three, but you shouldn't need a technical degree to be able to
understand the basic ideas.

The ultimate goal of the entire process is for the light that leaves
your monitor to produce the same perception as the light that
originally entered the camera would have if it had entered your
eyeballs instead. Alternatively, for images created with an
image-editing application, the goal is for your display to produce the
same perception (and basically the same light) as the artist's monitor
produced while he was creating the image. Clearly this involves both
the encoding process performed by the editor or conversion program that writes the
image file, and the decoding process, perfromed by the viewer or browser that reads
and displays the image, as well as aspects of human physiology and
psychology. We'll refer to the combination of the encoding and
decoding processes as the end-to-end process. PNG's role
is to provide a way to store
not only the image samples, that is, the color components of each pixel
but also the information needed to relate those samples to the desired output
of the display. A decoder that has both that information and knowledge of
how the user's display system behaves can then deduce how the image samples
must be transformed in order to produce the correct output.

Storing the image samples themselves is easy. The tricky part is figuring
out the two additional pieces of critical information: when encoding, how
the original light is related to the samples, and when decoding, how image
samples are related to the display's actual output (i.e., the reproduced
light). The fundamental problem is that working with and storing light is
nearly impossible; instead, light is typically converted to electrical signals.
Indeed, there are several more conversions along the way, each of which
potentially modifies the data in some way.

As a concrete example, in
an image captured via a video or electronic camera, light entering the camera
is first converted to analog voltages, which are in turn converted to other
voltages representing digital ones and zeros. These are stored in an image
file as magnetic fields on a hard disk or as tiny pits on a CD-ROM. For
display, the digital data in the file is optionally modified by the viewing
application (this is where gamma correction and other tweaking is performed),
then possibly converted again according to a lookup table (LUT),
then generally converted by a graphics card (``frame buffer'') back to
an analog electrical signal.[77]
This analog signal is then converted by the
monitor's electronics into a directed beam of electrons that excites various
phosphors at the front of the monitor and thereby is converted back into
light. Clearly, there is a bit of complexity here (no pun intended).

[77] Early PC graphics cards (the ``CGA'' and ``EGA'' adapters, for example)
communicated with the monitor digitally. Ironically, the burgeoning popularity
of flat-panel displays and digital television is driving manufacturers back
to using digital links between the frame buffer and display. As of early 1999,
the standards and products were rare to nonexistent, but they're coming.

But all is not lost! One can simplify this model in several ways. For
example, conversions from analog to digital and from digital to analog
are well behaved--they introduce minimal artifacts--so they
can be ignored. Likewise, the detailed physics of the monitor's
operation, from electrical signal to high-voltage electric fields to
electrons to light, also can be ignored; instead, the monitor can be
treated as a black box that converts an electrical signal to light in
a well-defined way. But the greatest simplification is yet to come.
Each of the conversions that remain, in the camera, lookup table, and
monitor, is represented mathematically by something called a
transfer function. A transfer function is nothing more than a
way to describe the relationship between what comes out of the
conversion and what went into it, and it can be a fairly complex
little beastie. The amazing thing is that each of the preceding
conversions can almost always be approximated rather well by a very
simple transfer function:

output = inputexponent

where the output and input values are scaled to the range between 0 and 1.
The two scaling factors may be different, even if ``input'' and ``output''
both refer to light; for example, monitors are physically incapable of
reproducing the brightness of actual daylight.
Even better, since the output of one conversion is the input to the next,
these transfer functions combine in a truly simple fashion:

This example happens to use three transfer functions, but the relation
holds for any number of them.
And the best part of all is that our ultimate goal, to have the final,
reproduced output light be perceived the same as the original input light,
is equivalent to the following trivial equation:

exponent1*exponent2*exponent3 = constant

Or in English: all of the exponents, when multiplied together, must equal a
single, constant number. The value of the constant depends on the environments
in which the image is captured and viewed, but for movies and slides projected
in a dark room, it is usually around 1.5, and for video images shown in typical
television or computer environments, it is usually about 1.14. Since the
viewing application has the freedom to insert its own conversion with its
own exponent, it could, in principle, ensure that the equation holds--if it
knew what all the remaining exponents were. But in general, it lacks that
knowledge. We'll come back to that in a moment.

In practice, images may be created with any number of tools: an
electronic camera; the combination of a classic film-based camera,
commercial developing process, and electronic scanner; an
image-editing application; or even a completely artificial source such
as a ray-tracing program, VRML browser, or fractal generator. To a
viewing application, a file is a file; there is rarely any obvious
clue as to the true origins of the image. In other words, the decoder
can have no reasonable expectation of divining any of the transfer
functions that came before the image data was saved to a file, even
if it asks the user for help. The decoder's sole concern must
therefore be the conversion of samples in the image file to the
desired output on the display.

We'll come back and deal with encoders in a little while. For a decoder there
are only two cases: either the file contains the additional information about
how the samples are related to the desired output, or it doesn't. In the
latter case, the decoder is no worse off than it would have been when dealing
with a GIF or JPEG image; it can only make a guess about the proper conversion,
which in most cases means it does nothing special.

But the case in which the file does contain conversion information is where
things finally get interesting. Many types of conversion information are
possible, but the simplest is a single number that is usually referred to as
gamma.
Gamma is a Greek letter
(γ)
that traditionally represents the exponent
in the first equation I gave; the only problem is that, as we've seen, there
are several exponents in the end-to-end process, and different people use the
term ``gamma'' to mean different things. I will use ``gamma'' to refer
to the exponent relating the image data and the desired display output.
Not surprisingly, this is how PNG's gAMA chunk defines gamma, too.[78]

[78] Version 1.0 of the PNG specification discussed gamma in terms of the
end-to-end transfer function from source to final display. This was
deemed impractical and not necessarily indicative of real-world
practice, so version 1.1 of the specification clarified all of the
gamma-related discussion and reserved the actual term ``gamma'' solely
for the usage described here.

10.2. The gAMA Chunk

PNG's gAMA chunk basically says: if your overall display system's
exponent (generally a combination of the system LUT exponent and the monitor
or CRT exponent) is the same as the inverse of this gamma value, then
the samples in the file are ready to go and need no further correction.[79]
If not, the decoding correction can be computed from the product of the overall
display-system exponent and the stored gamma value.

[79] Practically speaking, values that are within about 5% of each other
may be considered ``the same.''

More precisely (and here we get into a bit of mathematics that will mainly be
of interest to application developers), the stored gamma value represents the
following relationship between the image samples and the desired output light
intensity:

image_sample = light_outgamma

or:

image_sample1 / gamma = light_out

Once again, bear in mind that light_out and image_sample are
scaled to the interval between 0 and 1; that is, if the sample depth is 8 bits,
the file samples range between 0 and 255, so image_sample is obtained
by dividing a given file sample by 255, in floating-point arithmetic.

The decoding_exponent is simply the gamma correction that the
application applies; the combination of the other two exponents is the
``overall display system's exponent,'' to use the language with which we
began this section. Notice that the preceding equation and the one before it
are very similar--in fact, they imply the following relationship between
the exponents:

(1 / gamma) = decoding_exponent * LUT_exponent * CRT_exponent

or, equivalently:

decoding_exponent = 1 / (gamma * LUT_exponent * CRT_exponent)

The gamma relationship given in English at the beginning of this section
simply says that if the product on the right side of this equation equals
one (which means decoding_exponent also equals one), then no further
conversion is necessary--the image samples are ready to go as is.
On the other hand, if the right-hand side of the equation differs from one,
then that value is decoding_exponent and is what the decoder uses to
correct the image samples before sending them to the display system:

display_input = image_sampledecoding_exponent

Note that this procedure applies to each red, green, and blue value in a
truecolor image or to each palette value in a colormapped PNG. But it does
not apply to transparency values in an image with an alpha channel or
a tRNS chunk; alpha samples are always assumed to be linear. Implementors
should also be aware that there is no need to perform a computationally
expensive exponentiation for every pixel in the image, or three times per
pixel for an RGB image! At most, there are only 65,536 possible sample
values (for a 16-bit grayscale or 48-bit RGB image) and usually no more
than 256, which means that gamma correction can be accomplished via a simple
lookup table computed when the gAMA chunk is read.

That brings us to the gAMA chunk itself. Its contents are
quite simple: a 4-byte, unsigned integer equal to gamma multiplied by
100,000 and rounded to the nearest integer. So if gamma is
1/2.2 (or 0.45454545...), the value in the gAMA chunk is 45,455. There can
be only one gAMA chunk, and it must appear before any IDATs and also before
the PLTE chunk, if one is present.

As a practical matter, there is one more piece to the decoder half of the
gamma puzzle. The issue of exponents for the lookup table and monitor on
various systems is more complex than it should be, mainly because different
systems use the term ``gamma'' in strange and sometimes sneaky ways.
Table 10-1 summarizes the issue for some common platforms.

Table 10-1.Gamma Comparison Across Common Platforms

Platform

LUT_exponent

DefaultLUT_exponent

CRT_exponent

DefaultgAMA

PC

1.0

1.0

2.2

45,455

Macintosh

g/2.61

1.8/2.61

2.2

65,909

SGI

1/g

1/1.7

2.2

77,273

NeXT

1/g

1/2.2

2.2

100,000

The key thing to note, aside from the differences in default gAMA
values across platforms, is that both Mac OS and SGI IRIX allow the
user to modify a ``system gamma'' setting that not only differs from
the gamma definition we're using but also differs between platforms.
These ``gamma'' values modify the lookup table, and SGI's is
straightforward: LUT_exponent is simply the inverse of the SGI
``gamma'' value, which is denoted g in
Table 10-1. (NeXT workstations use the same convention as SGI, but the only way to
modify their setting is with third-party utilities.) The Macintosh,
on the other hand, not only defines its ``gamma'' as directly
proportional to LUT_exponent
but also divides it by a constant factor (2.61). Thus, while the
default Macintosh ``gamma'' of 1.8 appears close to SGI's default of
1.7, the actual lookup table exponents corresponding to these defaults
are 1.8/2.61 and 1/1.7, respectively.

10.3. Encoding Gamma

That wraps up gamma correction on the decoding side of things, but
what about encoders? After all, they must put the proper information
into the PNG file in the first place, so that decoders can do their
job correctly. The issue is more complex than for decoders, and not
only because there are so many ways to generate an image. Consider
the process of creating an image in an editor, which might seem the
most straightforward case since it involves, in some sense, exactly
the opposite procedure from that employed by the decoder. That is,
the artist manipulates the image so that the displayed output has the
desired appearance, then saves the result in a file with the proper
gamma. Ordinarily, the editing application would simply write a gamma
value that corresponds to the artist's display system. But if the
image in question originated on another system, some editors
will actually preserve its gamma setting by using a
decoding_exponent for all manipulations on the artist's
system--just as a normal viewer would. Thus the artist sees an
image displayed in her own ``gamma space,'' but the underlying image
samples actually remain in the gamma space of the original system.

The case of an electronic camera that writes image files directly turns out
to be the simplest possibility; as noted earlier, the camera has its
own transfer function and exponent, and the camera's manufacturer should
know precisely what that exponent is. When the camera saves an image,
whether in PNG format or something else, the proper gamma value is simply
the one that will make the end-to-end product of exponents equal to the
correct constant--which, you'll recall, is around 1.14 in the case of
images captured in a TV studio environment and intended for display on a
computer system. But even under different lighting conditions, the camera
knows what the conditions are and can correct for them accordingly,
perhaps via preset gamma settings for half a dozen situations, for example:
dimly lit, flash-illuminated, studio lighting, sunny day (high contrast),
bright cloudy day (lower contrast), and so on.

For images captured with a traditional camera and scanned from a
print, the issue is slightly fuzzier. If the scanner writes directly to
an image file with no user control of brightness and contrast, the case
is exactly analogous to that of the electronic camera: the scanner
manufacturer knows what its transfer function is and can encode the proper
gamma value in the file. But most scanners operate in conjunction with
editing software that allows the user to tweak not only gamma-related
settings but also color balance and saturation; this case is more like
the first one considered (regardless of whether the user considers
himself an ``artist'').

Ironically, images that are generated completely artificially are the
most complicated case. Most calculations on artificial scenes,
including those for VRML and ray-traced worlds, are done with ``linear
lighting'' that would correspond to a gamma of 1.0. But in creating
the scene, the artist usually makes adjustments based on how it
displays on her system, and if she happens to use a viewer that
performs no gamma correction, her feedback to the software that
generates the images will be skewed--in effect, she will modify the
colors, textures, lighting, and so forth, so that the gamma value
corresponds to her display system. The solution, of course, is to use
only software that supports gamma correction, both for generating the
images and for viewing them.

10.4. Gamma Gotchas

Finally, as a prelude to the following sections, I'll note a few
caveats. First, although I've referred to cathode-ray tube monitors
(or CRTs) throughout the discussion so far, not all computers use
them; in fact, notebook computers have long used liquid crystal
displays, and LCDs are becoming increasingly popular on desktop
systems as lightweight and space-saving alternatives to traditional
monitors. Do the simple exponential (or power-law) transfer functions
used earlier apply to LCDs as well? Yes, they do, but I need to
qualify that answer. Raw LCDs are actually characterized by an
S-shaped transfer function technically referred to as ``sigmoid'', for
which the best exponential fit would have an exponent of 1.0. This is
a lousy approximation, but fortunately, all real-world LCDs have
corrective circuitry built in that makes them behave like monitors.
So it is safe to use the same exponential transfer functions we
discussed earlier. If the extra circuitry did not exist, the
only reasonable-looking alternative would require support from both
the encoding and decoding software. Specifically, an image editor
running on an uncorrected LCD would need to include with the image a
full International Color Consortium profile, which we'll discuss at
the end of this chapter, and the decoder would in turn need to use it
to correct the image on other display systems. Alternatively, the
editor could precorrect the image samples to correspond to a normal
CRT and include only gamma information, but this would be a lossy
transformation of the image data.

A second caveat is that even when a monitor is the primary display device,
other output devices such as grayscale or color printers are often used
as well. Because of the vast differences in physics and technology between
an image reproduced by emitting light directly from a monitor versus one
reproduced as light reflected from printed paper, gamma correction is often
of lesser relative importance than color correction. A full color management
system may no longer be merely desirable but actually necessary. On the
other hand, printers are sometimes calibrated to work properly with the
local display, so an image that is gamma-corrected to look good on the
monitor will also print properly.

A third caveat is that monitors are not perfectly described by exponential
transfer functions, either. A better approximation is a combination of a
linear function near zero and an exponential function elsewhere. But a simple
exponential works well enough for most purposes.

The last thing to note is that even experts do not always agree, and
the issue of what exponent to use to describe CRTs is one of those
areas of disagreement. We've used 2.2 in the preceding discussion;
that's the value used in the sRGB specification (more on that later)
and the consensus of the color experts in the PNG Group. It is also
the value used by manufacturers of professional, calibrated display
equipment, such as Sony and Barco. On the other hand, Charles
Poynton, one of the Web's leading color experts and the author of a
number of technical papers and books, steadfastly maintains that 2.5
is more correct. At the time of this writing, things seem to be at an
impasse, but there is hope for resolution as further test results
become available in 1999.

In the meantime, Michael H. Brill has taken the initiative and written
a poem that not only summarizes the gamma disagreement rather nicely
but also does so with enviable wit and succinctness. It rhymes, too.
The poem is entitled "Gamma and Its Bases" and may be found on
the PNG home site: http://www.libpng.org/pub/png/book/gamma-poem.html.

10.5. Chromaticity

Adjusting the overall brightness of an image via gamma correction is a
good first step, but it does not address the issue of color balance. Anyone
who has visited a typical consumer electronics store has probably noticed
that not every model on the wall of televisions displays the same way. Some
may have a reddish tinge, some green; some usually display very
bright, saturated colors, while others may opt for slightly paler but more
realistic hues. Although one rarely sees a corresponding wall of computer
monitors and LCDs all displaying the same image, there are similar differences
between various manufacturers' models and even between monitors in the same
production run.

The main contribution to such variations comes from the manufacturers' choices
of light-emitting chemicals (phosphors) in monitors and of filters used in
liquid crystal displays. In addition, some higher-end monitors (and all color
TVs) allow one to adjust the color balance manually in one or more ways. The
details are not particularly important; what matters is that there
are differences--or to put it another way, the RGB color space is
device-dependent. Understanding how one quantifies and corrects for
these differences is most easily accomplished via a diagram.

Figure C-2 in the color insert, reproduced
in grayscale as
Figure 10-1, shows an interestingly
shaped color blob with a numbered curve and a brighter triangle embedded in
it and some numbers around its curved edge. The blob represents the complete
range of hues and saturation levels that the human eye can discern; a true
spectrum would wrap around the numbered edge[80]
(albeit without the cyan region near the upper left). The middle is composed
of smoothly interpolated mixtures, including ``white.'' The numbers on the
axes give the x and y
values of each hue and are directly related
to the International Commission on Illumination's (CIE, for Commission
Internationale de l'Éclairage) XYZ color space, a standard and
device-independent color space for well over half
a century. We'll come back to that shortly.

[80] The numbers give the wavelength (in nanometers) of the spectral colors
along the edge. Visible light lies within the range 400 nm to 700 nm,
roughly.

The brighter triangle in the middle represents the colors that can be
displayed by a particular monitor (not including any brightness information)
and is known as the color gamut of the display. The corners of
the triangle give the maximum-intensity red, green, and blue hues;
these directly correspond to the physical characteristics of the
phosphors used in the display. LCDs, printers, color film, and even
textile dyes have similar gamuts, though not always triangular.
Perhaps the most striking feature is the fact that the monitor's gamut
covers less than half of the complete color range. In other words,
there are many colors that the human eye can perceive but that cannot
be correctly represented on a monitor. The fact that the
chromaticity diagram can be displayed on a monitor at all means that
the region outside the triangle can be represented in some
manner, just not the correct one. This is the source of the cyan
error noted previously.

Because the diagram has been projected down from a three-dimensional color
space (XYZ) to the two-dimensional xy plane, information about the
relative intensities of red, green, and blue has been lost. That is,
the x,y values for the red phosphor indicate what color it emits at
any given intensity level and similarly for the green and blue phosphors.
But we still need to know the relative intensities of the three phosphors
when they are all at full power. This is where the concept of ``white''
comes in. In fact, there are many candidates for ``white,'' from the warm,
yellowish whites produced by incandescent lightbulbs to the cool, bluish
whites of electrical arcs and lightning.[81]
The curved line in the middle represents all possible values of ``white'' for
a given monitor, only one of which will be displayed as such. The associated
numbers along the curve refer to the ``blackbody temperature'' or color
temperature of any given white value; among other things, a star whose
surface (photosphere) is at the given temperature will emit light of the
given color most strongly.
[82]
Our Sun's surface temperature is around 6,000 degrees
Kelvin, for example; not coincidentally, this is the color temperature most
humans associate with ``average'' or ``true'' white.

[81] It is slightly odd that humans perceive redder light as ``warm'' and
bluer light as ``cool'' when, in fact, the opposite is true. Lightning
is far hotter than the filament in an incandescent bulb.

[82] Keep in mind that we are still talking about human perception.
A blackbody emits a true continuum of light; a monitor emits a more
limited continuum composed of three broad, overlapping
curves--corresponding to the red, green, and blue phosphors. Humans
perceive the monitor's ``white'' output to be the same as that of a
blackbody at a particular temperature, but a spectrometer would say
otherwise.

How does all of this relate to color correction in PNG? If the encoding
software knows the locations of the three corners of the triangle (the
primary chromaticities) and of white point,
it can save these values in PNG's chromaticity chunk, cHRM. When the image
is decoded on another system with a different color range, the decoder can
convert the x,y chromaticity values of both systems into XYZ space,
calculate any necessary adjustments between the two, and use that calculation
to convert the RGB values of the image into XYZ space and then into the RGB
space of the display system.

The simple way to deal with such conversions is to feed the
information to a color management system (CMS), assuming one is
present. All of the tricky details of conversion between different
color spaces and of mapping different monitor gamuts are handled by
the CMS. Color management systems are not yet in wide use on
typical users' platforms, however; a decoding application that
wishes to maintain optimal color fidelity will need to handle the
conversions on its own. The calculations to do so are not terribly
difficult, but they do involve a number of matrix operations. These are
detailed in of the University of Manchester's excellent
tutorial, Colour in Computer Graphics, and also in the "Color
Tutorial" section of the PNG Specification, Version 1.1.

Each of the eight values is an unsigned long integer, equal to the actual
floating-point value multiplied by 100,000 and rounded to the nearest integer.
Like the gAMA chunk, cHRM must precede all IDAT chunks and, if present, PLTE;
only one cHRM chunk is allowed.

10.6. Color Management Systems and sRGB

The popularity of the RGB color space is at odds with its fundamentally
device-dependent nature. In order to address this problem, a number of
manufacturers of computer-related equipment and the International Color
Consortium have cooperated to define a standard RGB space to which
various devices such as monitors, printers, scanners, and electronic cameras
can be calibrated. This specification, known as sRGB, is expected to be
approved as an international standard by the International Electrotechnical
Commission (IEC) by mid-1999; it will formally be known as IEC 61966-2-1.

sRGB allows one to create a PNG image on one system and print or display
it on another with full color fidelity and
without ever converting to XYZ
or another device-independent color space. How well it works in practice
remains to be seen, but a well-specified international standard--and
manufacturers' evident interest in it--will go a long way toward ensuring
that future devices are compatible at the RGB level.

In addition, an image that was created under sRGB can be flagged as such
with very little overhead. Only one parameter, the rendering intent, is
required; it is stored as a single byte in PNG's sRGB chunk. The rendering
intent, also known as ``artistic intent,'' indicates how the creator of the
image wishes the colors to be mapped when the output device's
color gamut (recall the discussion in the previous section) does not match
that of the original device. For example, imagine that an artist creates an
image on an sRGB-compliant monitor and graphics system, and when he's finished
he sends it to an sRGB-compliant color printer. Because the light-emitting
phosphors of the monitor and the light-reflecting inks of the printer and
its paper will be able to represent somewhat different ranges of
colors--ideally, mostly overlapping, but conceivably with only a little
overlap--it is necessary for the artist to specify how he wishes the
different color gamuts of the devices to be mapped to each other.

The simplest rendering intent (in concept) is known as absolute
colorimetric. The word ``colorimetric'' means color-measuring, and
this intent indicates that, for the region of overlap between source
and destination gamuts, any given pixel will be measured to have
identical colors on the two devices. When the output device is not
capable of reproducing some of the colors of the input device (i.e.,
the gamut is more restricted in that region of color space), the
colors are clipped to the nearest color that can be reproduced. The
result is that dynamic range will be lost in some areas. For example,
suppose that the image has a smoothly varying blue gradient and that
the output device is restricted to only the darker blues. The output
will show a smoothly varying gradient progressing from darkest blue to
medium blue, but then it will saturate and render all of the remaining
gradient as a constant, medium blue. Likewise, the intensity range
may be clipped if the output device is incapable of rendering absolute
black or the brightest shades of white. This rendering intent might
be used in cases in which three or more profiles are involved--for
example, when an image created on a computer display is intended for a
particular typesetter but first needs to be proofed on a local
printer.

A similar intent is relative colorimetric. As with the absolute flavor,
RGB values correspond to precise CIE color measurements, but they are modified
according to the intensity range and color cast (i.e., the white point) of the
output medium. Referring to our artist again, his monitor may be capable of
displaying true, 5,000K CIE white, but the paper in his printer generally will
not uniformly reflect all of the wavelengths that hit it, regardless of the
source.[83]
To put it another way, the paper will have a different white point than the
monitor.
As a result, it may be desirable to sacrifice perfect color correspondence in
favor of a similar dynamic range in intensities, by referencing the RGB values
to whatever paper or other output medium is used. The output image may have
an overall lighter or darker appearance or an overall color shift, but there
will be no clipping of grayscale gradients, and the colors will appear
to match--thanks to the human visual system's tendency to acclimate to an
overall tint or, to put it another way, to the ``prevailing white''. The
relative colorimetric intent is the ICC's default; it might be desirable for
displaying and printing corporate logos.

[83] And if he's silk-screening white T-shirts, no amount of bleach will change
that. There are some detergents that infuse clothing with small amounts of
phosphorescent chemicals in order to make ``whites whiter''; one's
clothes are no longer strictly reflective, but actually glow slightly when
exposed to blue or ultraviolet light. Such detergents are generally not
part of an sRGB-compliant display system.

A still more approximate intent, but one that may capture more of the
personality of the original image, is the perceptual rendering
intent. The idea in this case is to map the full color ranges of source and
destination devices as well as possible. This may involve either expansion,
compression, or shifting of the color gamut. Even colors within the region
where the gamuts overlap may be modified; in other words, absolute color
fidelity is less important than preserving the dynamic range in both color
and intensity of the image. This is often the most appropriate intent for
rendering photographs.

Finally we have the saturation-preserving rendering intent, which is
similar to perceptual rendering in that it doesn't necessarily enforce
completely accurate color reproduction. But rather than favor overall
gamut mapping like the perceptual intent does, this rendering intent specifies
that the saturation of each color should remain constant. Saturation can be
thought of as the amount of gray in a color of a given hue (say, greenish-aqua)
and lightness. As the saturation approaches zero, the color approaches gray;
maximum saturation gives the purest shade of the given hue. Since a cheap
inkjet printer might have only two-thirds of the saturation range of an
expensive dye-sublimation printer, colorimetric rendering might induce another
kind of clipping in the inkjet's output. Saturation-preserving rendering
would avoid that, but could possibly result in changes in hue and/or lightness. It
might be the preferred intent for printing business charts and graphs.

PNG's sRGB chunk encodes the rendering intent with the same values specified
by the International Color Consortium for ICC profiles: that is, byte value 0 for
perceptual, 1 for relative colorimetric, 2 for saturation-preserving, and
3 for absolute colorimetric.

Because the sRGB color space encompasses gamma and chromaticity information, it
is not strictly necessary for a PNG image to include gAMA and cHRM chunks in
addition to the sRGB chunk. But since not all applications will know how to
interpret sRGB, encoders should nevertheless include a gAMA chunk that
corresponds to sRGB, and possibly a cHRM chunk as well. Decoders that
know how to deal with cHRM are likely to know how to deal with sRGB, too,
which is why cHRM may be omitted. The proper values for the two chunks
are in
Table 10-3.

An sRGB-aware decoder should ignore gAMA and cHRM whenever an sRGB chunk is
present; the latter takes precedence. Less sophisticated applications can
use gAMA and cHRM to render the image approximately as intended, even without
knowledge of the sRGB color space. But note that there is no excuse for any
application written after the PNG 1.1 specification not to recognize
sRGB, at least; it is now part of the core spec, and new applications should
know what gamma and chromaticity values correspond to it, regardless of whether
the corresponding chunks--or even conflicting chunks--are actually present
in the file. As with gAMA and cHRM, only one
sRGB chunk is allowed, and it
must appear before any PLTE and IDAT chunks.

Table 10-3.sRGB Gamma and Chromaticity Values

gAMA

Image gamma

45,455

cHRM

White point x

31,270

White point y

32,900

Red x

64,000

Red y

33,000

Green x

30,000

Green y

60,000

Blue x

15,000

Blue y

6,000

10.7. ICC Profiles

For ultimate control over color fidelity and issues of device dependence, PNG
supports the ability to embed a full International Color Consortium profile
via the iCCP chunk. The ICC profile format, at version 3.4 as of this
writing, is a relatively mature specification that is itself headed toward
ISO standardization. The format is capable of describing not only computer
monitors, but also printers, scanners, liquid crystal displays, film,
transparencies, and so forth.

Though the profile format itself is understandably quite complex,
given all of the devices and color-space conversions it must
encompass, the format of PNG's iCCP chunk is independent of all
that. Similar to the zTXt chunk (which will be described in
Chapter 11, "PNG Options and Extensions"), iCCP contains only four elements, as shown in
Table 10-4: a printable name
terminated by a null byte; a byte indicating the compression method; and the
compressed profile itself.

Table 10-4.iCCP Chunk

Field

Length and Valid Range

Profile name

1-79 bytes (Latin-1 text)

Null separator

1 byte (0)

Compression method

1 byte

Compressed ICC profile

n bytes

The profile name is for the convenience of the artist or user of the image;
in practice, it will probably be similar to the profile description tag, which
is embedded in the profile itself. The compression method byte
currently must be zero, indicating a compressed stream in zlib format, using
the deflate compression method. As with zTXt and the actual image data, a
future major revision of the PNG spec may define other compression methods,
in which case this byte will be allowed to take on other values.

Aside from uncompressing it, ordinary decoders will not be expected to know
anything about the ICC profile other than the fact that they can be large
(i.e., more than 64 KB); instead, they will simply hand it off to the local
color management system for appropriate processing. Encoders should ensure
two things: that the profile is a valid ICC profile and that it
refers either to an RGB color space (for color images, including colormapped
ones) or to a grayscale color space. CMYK color spaces, for example, are
disallowed. Likewise, multiple copies of iCCP are disallowed; if the iCCP
chunk is present, it must come before any PLTE or IDAT chunks.

By mid-1998, there were indications that something of a ``TIFF effect''
applied to the ICC profile format; that is, profiles from different vendors
were not necessarily interoperable with each other or with different color
management systems.[84]
Presumably this will be worked out by the time the ICC specification becomes
an official standard, but in the meantime, it is something of which PNG
implementors should be aware.

[84] This is hardly surprising for a format that attempts to deal with
such a thorny problem.